In general, polarizing plates have a stacked structure constituted by a polarizer formed of a polyvinyl alcohol (PVA)-based resin and dyed with dichroic dyes or iodine, and a protection film attached to one or both sides of the polarizer using an adhesive. In the related art, triacetyl cellulose (TAC)-based films have mainly been used as protection films for polarizing plates. However, TAC films are easily deformed in high-temperature or high-humidity environments. Therefore, protection films made of various types of material have recently been developed to replace TAC films. For instance, methods of using polyethylene terephthalate (PET), cycloolefin polymer (COP), acrylic films, or combinations thereof have been suggested.
Such films for protecting polarizers may include various functional coating layers, such as reflection preventing layers and hard coating layers formed on sides opposite to polarizers for the purposes of preventing reflections, improving durability, preventing scratches, and improving visibility. Such functional coating layers are generally formed by applying coating compositions including base resins, solvents and additives to protection films, and then curing the coating compositions. However, functional coating layers may not adhere sufficiently to some kinds of protection films. In such cases, functional coating layers may be separated from the protection films or damaged, thereby lowering the performance of polarizing plates.
Although methods of performing surface treatments such as a plasma treatment and a corona treatment on protection films or forming primer layers on protection films have been suggested to solve such problems, such methods remain insufficient to secure reliable adhesion of functional coating layers and protection films (particularly acrylic protection films). In the related art, for instance, urethane primers suggested as primer layers for protection films have low water resistance and solvent resistance. Therefore, if urethane primers are stored under high-moisture conditions for a long period of time, adhesive strength of the urethane primers may be lowered by water penetration, and if coating compositions including organic solvents are applied to primer layers, the primer layers may be swollen by the solvents included in the coating compositions or may be separated from protection films while being dissolved in the coating compositions.
Therefore, there is a need for primer compositions having high water resistance and solvent resistance for polarizing plate protection films.